1. Field of the Invention
The invention in general relates to electronic oscillators and more particularly to a crystal oscillator utilizing a plurality of piezoelectric crystals.
2. Description of Related Art
A crystal oscillator is an electronic circuit that produces electrical oscillations at a particular designed frequency determined by the physical characteristics of one or more crystals, generally of quartz, positioned in the circuit feedback loop.
During operation of the crystal oscillator, the output frequency may drift relative to the designed frequency, necessitating a tuning operation to bring it back. For this purpose a tuning mechanism can be provided. Crystal oscillators are conventionally tuned by inserting a variable reactance circuit in series with the crystal. Electrical tuning is normally accomplished using variable capacitance (varactor) diodes to provide the variable reactance. A typical crystal oscillator utilizing varactor diodes for tuning is described in U.S. Pat. No. 4,851,790.
The tuning range of a typical crystal oscillator is however limited to a small tuning range such that it may not be possible to bring the frequency back to the desired value if the output frequency drifts too far.
Primary drift mechanisms are associated with changes in crystal environmental stress (temperature, pressure, etc) and also with long-term aging. Long-term aging is a particular problem in crystal oscillators designed to produce a low noise output signal. This is because aging is more severe in crystals necessarily operated at high drive levels in order to achieve a high signal-to-noise ratio. If the crystals age too far off frequency, the circuit may not be able to tune back to the designed frequency, due to the limited tuning range associated with such crystal oscillators.
It is therefore a primary object of the present invention to provide a crystal oscillator having a tuning range significantly greater than that of typical prior art crystal oscillators.